Conventionally, gas turbine aircraft engines have the external power taken through a geared external drive from the high pressure rotor system. FIG. 1 diagrammatically shows such a gas turbine engine 10 with three drive shafts 12, 14, 16. The shaft 12 will be connected to the fan (not shown), whilst the shaft 14 is connected to the intermediate pressure compressor with a rotor 18. The shaft 16 is connected to the rotor 20 of the high pressure compressor. The intermediate casing and internal gearbox 22 is located between the rotors 18, 20. Downstream of the compressors are the combustor 24, high pressure turbine 26, intermediate pressure turbine 28, and low pressure turbine 30.
Having an external drive attached to the high pressure compressor system has a secondary function, enabling hand or ground cranking of the high pressure rotor 20 to enable inspection thereof through boroscope access ports in the casing.
The demand for electric power within aircraft is increasing and this makes it beneficial for power to be extracted from the intermediate power compressor system. With the intermediate power compressor, more power is available at low speed without compromising compressor stability. In view of this, proposals have been made to use a concentric high power/intermediate power external drive. With this system a switchable clutch type coupling has been provided to connect the intermediate pressure and high pressure spools for starter assist.
A feature of this arrangement is that the high power drive is permanently connected to the high power rotor, thereby providing a means for enabling rotation of the high pressure rotor for inspection. With this arrangement though, the high pressure gear is only required for starting but is permanently connected to the high pressure rotor, and therefore spins under normal running which is a source of windage loss and reduced reliability.
It may therefore be desirable just to provide a drive off the intermediate pressure shaft, but there is then the problem of enabling turning of the high pressure rotor to enable inspection.
Proposals have been made to enable a tool to engage with the high pressure spool gear by being inserted into the internal gearbox. It is however undesirable to break into a clean oil chamber with an external tool that could allow ingress of damaging dirt or grit. Other proposals have also been made, including for instance inserting a geared tool into the bearing chamber, but again there is the possibility of the ingress of dirt into the chamber. It is also necessary to provide a substantial access hole to enable a sufficiently robust tool to be used which could for example be used with a seized high pressure spool.